As modern electronic circuit boards become more and more complex with increased circuitry and component densities, thorough cleaning of boards after soldering becomes more and more important. In the manufacture of such printed electronic circuit boards, the components are usually fixed in place by soldering. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with fluxes which are commonly rosin acids used alone or with activating additives such as amine hydrochlorides or oxalic acid derivatives, and passing this coated side of the board over preheaters and then through molten solder. The flux cleans the conductive metal parts and promotes adhesion of the solder. The temperature used during soldering usually thermally degrades some of the flux leaving flux and flux residues on the board. These flux and flux residues may interfere with the electrical function of the circuit board and must be removed. The complexities of modern circuit boards are such that the only practical process for removing the remaining flux, flux residues and other foreign matters from the circuit board is by solvent cleaning.
The most commonly used current industrial process for cleaning circuit boards after soldering is by the use of vapor defluxing. In the conventional operation of a vapor defluxer, the soldered circuit board is passed through a sump of boiling organic solvent which removes the bulk of the residual flux (including thermally degraded flux) and thereafter through a sump containing freshly distilled solvent at near room temperature, and finally through solvent vapor over a boiling sump which provides a final rinse with clean solvent which condenses on the circuit board. In addition, the board can also be sprayed with distilled solvent before the final rinse.
Ideally, in the above vapor defluxing process, the solvent used would be a single pure solvent, but in practice it has not been possible to provide a single solvent with all the necessary characteristics of relatively low boiling point, nonflammability, low toxicity and high solvency for flux and flux residues. Therefore, in the art, it has been the practice to use a mixture of solvents to control these characteristics.
While solvent mixtures may be carefully tailored to control boiling, flammability and solvent power effectively, such a solvent mixture is not necessarily useful in industrially-used circuit board cleaning procedures such as vapor defluxing as described above. The major deterrent to the use of such solvent mixtures is that they fractionate to an undesirable degree during use. For example, in the vapor defluxing technique described above, the first stage in the cleaning consists of passing the soldered circuit board into a sump of boiling organic solvent. Under such conditions the lower boiling component of the solvent mixture may be vaporized leaving the higher boiling components of the mixture with altered solvency characteristics.
Again, in this cleaning procedure, the final rinse is carried out by passing the cleaned circuit board through solvent vapor over a boiling sump. This vapor condenses on the circuit board and any fractionation which may occur will alter its solvency characteristics. Finally, for the cleaning procedure to be economically feasible, the used solvent must be readily recoverable for reuse, usually by distillation. Again, the recovered solvent should have the same composition and characteristics as the original solvent system.
Thus, azeotropic solvent mixtures with their constant boiling and constant composition characteristics have been found to be very useful in the solvent cleaning process. Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes and therefore there is constant effort to discover azeotropes or azeotrope-like solvent mixtures with desirable characteristics for cleaning circuit boards.
One such azeotropic composition is disclosed by Burt in U.S. Pat. No. 3,455,835 wherein a composition of from about 54 to about 64 percent by weight of 1,1,2-trichloro-1,2,2-trifluorethane and from about 36 to about 46 percent by weight of trans-1,2-dichloroethylene boils at about 44.degree. C. at atmospheric pressure and is an effective cleaning solvent for circuit boards.
While the compositions as disclosed by Burt are effective solvent systems for cleaning circuit boards, the solvent systems should be stabilized against compositional and other changes during long term storage and prolonged use. Changes due to oxidation, polymerization, interaction of the components and the like may generate products which adversely affect the solvent composition itself and the performance of the circuit boards being cleaned.
It is, therefore, an object of the present invention to provide a stabilized azeotrope or azeotrope-like composition of 1,1,2-trichloro-1,2,2-trifluoroethane and trans-1,2-dichloroethylene. It is a further object of the present invention to provide a stabilized azeotropic composition of 1,1,2-trichloro-1,2,2-trifluoroethane and trans-1,2-dichloroethylene which minimizes corrosion of aluminum alloys and gel (polymer) formation.